kline@ux1.cso.uiuc.edu (03/05/90)
The idea: Run two-wire twisted pair throughout an installation with mini jacks in various locations. People wishing to communicate plug in an intercom module which is battery-powered and has a earphone and small microphone. The signal on the wires should be "additive" so that anyone talking loud enough to break squelch on their unit should add in to the signal on the wires, so everyone can hear everyone who is talking. Design problems: I don't know how to make a circuit which will drive the twisted pair in this "additive" manner. I'm using an LM386 amplifier to read the twisted pair and drive an earphone, which works fine, but using another LM386 driving the twisted pair through a transformer to match the high impedance of the twisted pair gives poor results, as does using a 741 driving the pair through a resistor. Stations on the wire interfere with one another, and I don't know how to solve this problem. It wouldn't be anything as simple as a diode, would it? I also have no idea where to begin on a squelch circuit (actually a voice-activated switch, I suppose). I've played around on paper with a high-pass filter to saturate a transistor which would then allow the transmitted signal to pass, but I'm really in the dark. Any help would be most appreciated. Or if anyone knows of a commercial product which does what I describe that's SMALL and <$20/station, that would work too. Although I still wouldn't mind some suggestions on how to solve these problems, just to satisfy my own curiosity. Thanks! _____ Charley Kline, University of Illinois Computing Services c-kline@uiuc.edu uunet!uiucuxc!kline